1. Field of the Invention
The present invention is directed to switched capacitor circuits formed from polysilicon thin film transistors (TFTs) and polysilicon thin film capacitors (TFCs), and in particular to switched capacitor analog circuits such as integrators, amplifiers, and digital-to-analog converters (DACs) using polysilicon TFTs and TFCs which can be used as analog driving circuitry for large area electronic (LAE) devices such as active-matrix liquid crystal displays, pagewidth optical scan arrays, or electrographic or ionographic printheads.
2. Background
Large area electronic devices usually consist of one- or two-dimensional arrays of thin-film circuit elements (often referred to as pixels). These pixels might contain, for example, liquid-crystal light valves for a display, or photodiodes for an optical scan array, or nibs for a print array. In each case the physical size of the array is determined by the application, and is much larger than a conventional silicon integrated circuit. The arrays are therefore built on large area substrates, usually of glass or quartz. The pixel arrays also require driving and interface circuitry, and in most cases this circuitry must be analog rather than digital, that is it must be capable of delivering or sensing a range of input signals. Suitable analog circuitry can be built using well-known switched capacitor techniques in conventional silicon integrated circuits (ICs). These ICs must then be mounted on or adjacent to the large area substrate containing the pixel array, and a large number of electrical connections must be made between the two. The cost of the peripheral drive and interface chips, their mounting and their electrical connection to the large area device can constitute a significant proportion of the overall cost of a system containing a large area device. If the ICs and connections can be eliminated or greatly reduced by integrating suitable circuitry on the large area substrate, then the system cost can be reduced and its reliability improved.
Thus, it is desirable to integrate drive circuitry with the pixel elements on a common substrate (e.g., glass or quartz). A number of circuits have been integrated with LAE displays using polysilicon and amorphous silicon thin film technologies. However, these have been purely digital circuits, or, where analog drive is needed, have used a simple pass transistor to deliver the analog signal to the array with the state of the pass transistor being controlled by digital circuitry.
It has been recognized that polysilicon thin film technology may enable the integration of drive circuitry on a substrate with a pixel array. However, due to the inferior performance of polysilicon thin film transistors (TFTs) when compared with conventional single crystal silicon MOS field effect transistors (MOSFETs), it has been thought that the fabrication of true analog circuits using polysilicon thin film technologies is not possible.
Polysilicon TFTs are inferior to conventional silicon MOSFETs in several ways. First, the electrical drive current available from a polysilicon TFT is much lower than that of a similar-sized MOSFET. This limitation also applies to digital circuits, but is more severe under the bias conditions typically employed in analog amplifiers. Second, the saturation characteristics of polysilicon TFTs are poor, with a low output impedance caused by the so-called "kink" effect which arises from channel avalanche multiplication and which is made worse by the presence of trap states in the device channel. This low output impedance is much more important for analog circuits than digital since it can limit the voltage gain available from an amplifier. Third, polysilicon TFTs are known to suffer from relatively high off-state leakage currents compared with MOSFETs. In analog applications, it is often necessary to store charge on a capacitor, and any charge lost due to TFT leakage will cause an error in the analog signal. Digital circuits, on the other hand, are much less susceptible to leakage; even in a dynamic design, where charge is also stored on a capacitive node, the total charge loss must exceed some threshold value before any signal error will arise, and this threshold is normally much larger than the acceptable charge loss in an analog circuit. Fourth, polysilicon TFTs exhibit much higher electrical noise then MOSFETs, a problem which is again much more important in analog applications than digital.
Single crystal thin-film technologies, also referred to as silicon-on-insulator technologies, such as silicon-on-saphire (SOS), separation by implanted oxygen (SIMOX) or zone-melt recrystallization (ZMR), also suffer some of the limitations discussed above, notably the kink effect (although it is not so severe in single crystal SOI MOSFETs as in polysilicon TFTs) and increased leakage. These technologies are not commonly used for analog applications, in part for these reasons.
A number of references recognize that it would be desirable to use TFTs to form integrated drivers for LAE devices such as liquid crystal displays. These references disclose polysilicon treatments which improve some of the characteristics of TFTs, however, even the improved polysilicon TFTs do not approach single crystal transistors in operating characteristics. Moreover, none of these references disclose switched capacitor analog circuits constructed using polysilicon thin film technology.
Alan G. Lewis and Richard Bruce, in "Circuit Design and Performance for Large Area Electronics", 1990 IEEE International Solid-State Circuits Conference, pp. 222-223, Feb. 16, 1990, discuss the use of polysilicon TFTs to form operational amplifiers (see FIG. 4). The use of cascodes (two or more transistors in series with separated gates) is disclosed in order to compensate for the kink effect in polysilicon TFTs. In spite of the low performance of polysilicon TFTs (low drive currents, higher threshold voltages), digital shift registers were fabricated which operate at high speeds (30 MHz). This is believed to be due to the low parasitic capacitance between the TFTs and the insulator substrate. Additionally, an operational amplifier constructed entirely from polysilicon TFTs is disclosed. This op-amp minimizes drain biases on the n-channel TFTs which are most susceptible to the degradation of output impedance due to the kink effect, and a complementary source-follower output stage is used to overcome the limited drive current. However, switched capacitor circuits were not demonstrated, and issues such as TFT leakage, compensation of the amplifier required for switched capacitor applications, and the linearity of thin film capacitors were not discussed.
S. N. Lee et al, in "A 5.times.9 inch Polysilicon Gray-Scale Color Head Down Display Chip", 1990 IEEE Solid-State Circuits Conference, pp. 220-221, Feb. 16, 1990, and R. G. Stewart et al in "A 9V Polysilicon LCD with Integrated Gray-Scale Drivers", pp. 319-322, SID 90 Digest, disclose a driver circuit for an LCD display which receives a digital input and an analog ramp-voltage input to produce an analog output to control the gray scale of the LCDs. All the polysilicon circuitry is digital; the bulk of it controls the time during which a pass transistor is held in the conducting state, and hence how much of the externally generated ramp is delivered to a given data line.
N. Yamauchi et al in "Drastically Improved Performance in Poly-Si TFTs With Channel Dimensions Comparable to Grain Size", IEDM, pp. 353-356, 1989, discloses processes for forming polysilicon TFTs which improve their field-effect mobility and current leakage.
D. M. Kim et al, in "Characterization and Modeling of Polysilicon TFTs and TFT-CMOS Circuits for Display and Integrated Driver Applications", SID Digest, pp. 304-306, 1990 disclose digital flip-flops, level shifter devices and buffer devices constructed from polysilicon TFTs.
K. Nakazawa et al in "Lightly Doped Drain TFT Structure for Poly-SiLCDs", SID Digest, pp. 311-314, 1990, discusses the promise of forming on-glass peripheral circuits made from polysilicon TFTs on full-color flat-panel liquid crystal displays. However, the low field-effect mobility and high current leakage are recognized as problems which still need to be overcome.
K. Ono et al in "Polysilicon TFTs With Low Gate Line Resistance and Low Off-State Current Suitable for Large Area and High Resolution LCDs", IEDM Digest, pp. 345-348, 1989, disclose polysilicon TFTs having lower gate line resistances and lower off-state currents by reacting Pt with gate polysilicon films.
Alan G. Lewis et al in "Physical Mechanisms for Short Channel Effects in Polysilicon Thin Film Transistors", IEDM Digest, 349-352, 1989, discuss the physical mechanisms responsible for short-channel threshold shifts in n- and p-channel polysilicon thin film transistors.
Japanese Patent Publication No. 59-182569 to Kumada discloses a thin film transistor having improved output characteristics by inactivating the surface of the thin film source and drain electrodes through heat treatment in a mixed gas containing hydrogen when the transistor is formed on an insulating substrate.
Wu et al in "Performance of Polysilicon TFT Digital Circuits Fabricated With Various Processing Techniques and Device Architectures", SID Digest, pp. 307-310, 1990, discuss fabrication processes for improving polysilicon TFTs, as well as digital driver circuits for LAE devices formed from these polysilicon TFTs.
Y. Matsueda et al in "New Technologies for Compact TFT LCDs With High-Aperture Ratio", SID Digest, pp. 315-318, 1990 disclose an LCD matrix wherein one storage capacitor line is provided for every two scanning lines.
Additional references which disclose TFTs used in digital circuits for LAE devices include U.S. Pat. No. 4,872,002 to Stewart et al, U.K. Patent Application No. 2,117,970 to Oshima et al, and Japanese Patent Publication No. 61-13665 to Hiranaka.
U.S. Pat. No. 4,872,002 to Stewart et al describes switched capacitor load circuits used in an integrated digital display driver. These load circuits are used to simulate a resistive load for TFT latches, and allow the gain of the latch to be modified. This load circuit does not include an amplifier, and is more simple than the circuits of the present invention, both in consisting of fewer elements, and in what it achieves. More complex circuits are not suggested. Stewart et al also note that their switched capacitor circuits deviate from ideal behavior (a feature of all switched capacitor circuits which limits their usefulness). It was expected that the analog circuits constructed according to the present invention would be fairly poor because of this characteristic of polysilicon switched capacitor circuits. However, surprisingly, they performed much better than expected.
U.K. Patent Application No. 2,117,270 to Oshima et al discloses digital circuits constructed from polysilicon TFTs.
The abstract "Charge-Sensitive Poly-Silicon Amplifiers For A-Si Pixel Particle Detectors", by Cho et al states that charge-sensitive poly-silicon TFT amplifiers have been made, but no details are given.
U.S. Pat. No. 4,783,146 to Stephany et al disclose TFTs used as switches in a liquid crystal print bar.
U.S. Pat. No. 4,772,099 to Kato et al discloses the use of polysilicon TFTs as switches in liquid crystal displays.
Additional background material relating to TFTs include "Depositing Active And Passive Thin-Film Elements On One Chip" by Harold Borkan, Electronics, Apr. 20, 1964 and "The TFT-A New Thin-Film Transistor" by Paul K. Weimer, Proceedings of the IRF, 1962.
The above references are incorporated herein by reference for purposes of background material regarding digital and analog driver circuitry.
While many of these references recognize the desirability of forming driver or interface circuitry from polysilicon TFTs because such circuitry can be integrated on the large area substrates which currently contain the arrays of LC light valves, photodiodes or print nibs, none of these references disclose the present invention. Many of the references seek to improve the performance of polysilicon so that TFTs function more like single crystal devices. Other references form digital circuits from polysilicon TFTs. However, none of the references disclose true analog circuits made from polysilicon, and in fact, the skilled artisan would not expect such circuits to achieve useful performance due to the limited performance of polysilicon TFTs discussed above.